Lecture # 17: Nervous Tissue

1. Lecture # 17: Nervous Tissue (Chapter 12) Objectives: 1- Explain the general role of the nervous system in maintaining homeostasis. 2- Name the subdivisions of the nervous system and list the components of each. 3- Identify the parts of a neuron and give their functions. 4- Classify neurons functionally and structurally. 5- Contrast characteristics and functions of neuroglia and neurons. 6- Identify and give the specific functions for the types of neuroglial cells. A Purkinje cell, a neuron from the cerebellum

2. Overview of the Nervous System There are two organ systems dedicated to maintaining the internal conditions (homeostasis): Endocrine gland Endocrine System It communicates by means of chemical messengers (hormones) secreted into to the blood Hormone Nervous System It employs electrical and chemical means to send messages from cell to cell Target cell (skeletal muscle cell) Neurotransmitter (Ex: Acetylcholine)

3. Anatomical Subdivision of the Nervous System • Brain • Central Nervous System (CNS) • Peripheral Nervous System (CNS) • Spinal cord • Nerve • Ganglion • It is all the nervous system except the brain and spinal cord. • It consists of the brain and spinal cord enclosed by cranium and vertebral column. • It consists of nerves and ganglia. It deliveries sensory information to the CNS and carries motor commands to peripheral tissues and system. It is responsible for integrating, processing and coordinating sensory data and motor commands. Nerve: It is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue in the PNS It is a bundle of nerve fibers (axons) in the CNS (white matter). Tract: • It is a knot-like swelling in a nerve where neuron cell bodies are concentrated in the PNS Ganglion: Nucleus: • It is a concentration of neuron cell bodies in the CNS (gray mater)

4. Overview of the Nervous System Sensory or Afferent Division It carries sensory signals from various receptors to the CNS Receptors Sensory input Central Nervous System Sensory or Afferent Division Peripheral Nervous System • The brain and spinal cord processes the information, relates it to past experiences, and determine what response is appropriate to the circumstances Motor or Efferent Division Effectors Motor output Motor or Efferent Division It carries signals from the CNS to gland and muscle cells that carry out the body’s response

5. Brain Visceral sensory fiber Sympathetic Spinal cord Parasympathetic Somatic sensory fiber Somatic motor fiber

7. -In the CNS -In the PNS Nervous Tissue 1- Neurons Glial cell (astrocyte) 2- Neuroglia or glial cells Neurons The glial cells are supporting cells, which are associated to the neurons and provide a supportive scaffolding for neurons Neuron Neurosoma, cell body, or perikaryon Dendrites Axon

8. Properties of Neurons Universal Properties Excitability (irritability) • They respond to environmental changes called stimuli by producing electrical signals (action potentials) Conductivity • The action potentials are quickly conducted to other cells at distant locations Secretion • When electrical signal reaches end of nerve fiber, a chemical neuro-transmitter is secreted that crosses the gap and stimulates the next cell Action potential Stimulus

9. Functional Classes of Neurons Sensory or afferent neurons They conduct signals from receptors to the CNS. Some receptors, such as those for pain and smell, are themselves neurons Receptors Sensory input Central Nervous System Interneurons • They occur only in the CNS. They receive signals from many other neurons; and process, store, and retrieve information and make decisions that determine how the body respond to stimuli Effectors Motor output Motor or efferent neurons They conduct signals from the CNS to the effectors (gland and muscle cells )

10. Functional Classes of Neurons

11. Structure of a Neuron Nissl bodies Dendrites They receive nerve impulses from other neurons Soma Nucleolus Nucleus Axon collateral Axon hillock Axon It carries the nerve impulse away from the soma. From 1 to 20 mm in diameter and from a few mm to more than one meter long Node of Ranvier Schwann cell They produce the myelin sheath They carry on the synthesis of protein Neurofibrils Terminal arborization It is the trigger zone for the nerve impulse They release the neurotransmitter Synaptic knobs

12. Structure of a Neuron Axoplasm Schwann cell nucleus Dendrites Cytoplasm of axon Neurilemma They receive nerve impulses from other neurons Axolemma Thick outermost coil of myelin sheath Soma Plasma membrane of axon Axon collateral Axon It carries the nerve impulse away from the soma. From 1 to 20 mm in diameter and from a few mm to more than one meter long Myelin sheath • It consists of the plasma membrane of glial cells (20% protein and 80 % lipid). It electrically insulates the axon Node of Ranvier Schwann cell They produce the myelin sheath Terminal arborization Synaptic vesicles They contain the neurotransmitter They release the neurotransmitter Synaptic knobs

13. Structural Classification of Neurons • They have one axon and multiple dendrites. They are the most common neurons in the brain and spinal cord. • They have one axon and one dendrite. Ex: Olfactory cells, retina, inner ear. • They have single process leading away from the soma. A short distance away from the soma, the process branches like a “T”. They carry sensory information from skin and organs to spinal cord. • They have many dendrites but no axon. They are found in the brain, adrenal medulla, and retina where they help in visual processes

14. Supportive Cells (Neuroglia) Central Nervous System Oligodendrocytes • 2- Ependymal cells: They form the ciliated cuboidal epithelium that lines internal cavities of the brain. They secrete and circulate cerebrospinal fluid (CSF Ependymal cells Supportive Cells (Neuroglia) Microglia Astrocytes Peripheral Nervous System Schwann cells Satellite cells Neuroglia in the CNS: • 1- Oligodendrocytes: They form myelin sheaths in CNS. Each arm-like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction • 4- Astrocytes: They are the most abundant glial cell in CNS covering the entire brain surface. They have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood-brain barrier. • 3- Microglia: They are small, macrophages formed from white blood cell called monocytes. They wander brain tissues ingesting microorganisms and dead tissue. The blood-brain barrier isolates the blood from the brain tissue and limits what substances are able to get the brain cells, thus protecting the neurons. • Astrocytes regulate the chemical composition of tissue fluid by absorbing excess neurotransmitters and ions. Astrocytosis or Sclerosis: When the neurons are damaged, the astrocytes form hardened scar tissue and fill space formerly occupied by the neuron.

15. Neuroglia in the PNS: • 1- Schwann cells: They wind repeatedly around a nerve fiber and produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS. Schwann cells assist in the regeneration of damaged fibers. Clinical Application: • 2- Satellite cells: They surround the neurosomas in ganglia of the PNS and provide electrical insulation around the soma. Satellite cells regulate the chemical environment of the neurons. • Tumors: They are masses of rapidly dividing cells. Mature neurons have little or no capacity for mitosis and seldom form tumors. • Brain tumors arise from: • - Meninges (protective membranes of CNS). • - By metastasis from non-neuronal tumors in other organs. • - Most come from glial cells (gliomas) that are mitotically active throughout life. • Gliomas grow rapidly and are highly malignant. The blood-brain barrier decreases effectiveness of chemotherapy. The treatment consists of radiation or surgery.